The present invention relates to a method and a device for exposing a sheet to electromagnetic radiation.
There is a well known method of performing thermographic duplication, in which a heat sensitive sheet, which is affected by heat and which is substantially transparent, is superposed over an original bearing a pattern to be copied which is formed of an infrared radiation absorptive material, and electromagnetic radiation, preferably electromagnetic radiation which contains a high proportion of infrared radiation, is radiated onto this superposition of the heat sensitive sheet over the original. This electromagnetic radiation penetrates through the substantially transparent heat sensitive sheet onto the original, and those parts of the heat sensitive sheet which are resting against the patterns of the original are heated by heat which is generated in these patterns by absorption of infrared rays from the electromagnetic radiation. Therefore, the parts of the heat sensitive sheet are changed, for instance by change of color, perforation or shrinkage. Thus, the heat sensitive sheet picks up an image thereon which is a copy of the pattern on the original sheet.
For this kind of thermographic copying, it is very important for the heat sensitive sheet to be uniformly pressed against the original, in order that the heat generated in the original pattern by the electromagnetic radiation is effectively transferred to the corresponding portions of the heat sensitive sheet. For this pressing, strong pressure needs to be applied to the superposed heat sensitive sheet and the original bearing the pattern to be copied, during the process of irradiation thereof by electromagnetic radiation. More particularly, in such a copying process, there needs to be applied a certain minimum pressing force per unit area of the superposition of the heat sensitive sheet and the original, for satisfactory copying.
Thus, if the original is fairly small, the provision of this pressing force per unit area is not a great problem. However, for an original which is rather large in size, the provision of thus fairly substantial pressing force per unit area, smoothly and uniformly over the entire surface of the superposition of the heat sensitive sheet and the original, becomes very difficult, because the total force to be applied becomes very high. For example, a typical value of pressure which is required between the heat sensitive sheet and the original is of the order of 200 g to 300 g per square centimeter, and, for a B4 size original, the total pressing force which neds to be applied is therefore of the order of 200 Kg to 300 Kg, which is a very high force. Of course, as the size of the original further increases, the difficulty of applying the total required pressing force between the heat sensitive sheet and the original proportionally increases.
Another problem which occurs, when the size of the original is increased with conventional thermographic devices utilizing the process outlined above, is that there is a certain optimum amount of heat energy per unit area of the original that should be available for transfer to patterns of the original. That is to say, in the case of using a flash discharge tube such as a xenon lamp as the source of the irradiating electromagnetic radiation, so as to carry out the thermography operation instantaneously by a flash light, if the original is quite small in size, one xenon flash discharge tube will be sufficient, but if the size of the original is increased a plurality of powerful high output type xenon flash discharge tubes, and consequently a bulky system of capacitors, will be required. This problem, again, increases in direct proportion to the increase in size of the original to be copied.
Therefore, as the size of the original which is to be copied increases, the power output of the electromagnetic source for irradiating the superposition of the heat sensitive sheet and the original, and also the total pressure which must be exerted on the superposition increase greatly, and therefore the cost and the massiveness of such a device become quite unrealistic.